1. Field of the Invention
The invention relates to a vehicular automatic transmission. More specifically, the invention relates to an improvement in a neutral control that interrupts the transmission of power when driving while an automatic transmission is in a running speed.
2. Description of the Related Art
A known neutral control apparatus for a vehicular automatic transmission includes (a) an automatic transmission in which a shift is achieved by the application of a predetermined engaging element, and (b) neutral control executing means for executing a neutral control that interrupts the transmission of power by either releasing the engaging element, or allowing it to slip, when a predetermined condition to execute the neutral control is fulfilled when driving while the automatic transmission is in a running speed. One example of such an apparatus is disclosed in JP(A) 4-77828. The disclosed apparatus executes a neutral control while the vehicle is stopped in order to both prevent the generation of creep torque and improve fuel efficiency by reducing the engine load. When the hydraulic fluid temperature is so high that the volumetric efficiency of the pump is reduced, however, the apparatus cancels the neutral control to facilitate responsiveness during torque restoration.
A friction apply device which generates application force by friction is typically used as the engaging element, and slip control (e.g., a state in which the clutch is partially applied) is frequently executed during neutral control in consideration of responsiveness and shift shock and the like during torque restoration. This kind of slip control, however, generates a heat load on the friction apply device. If great enough or sustained long enough, the heat load may impair the durability of the friction apply device. Although it is possible to improve the durability of the friction apply device by, for example, increasing the number of friction plates lined with friction material, increasing the heat-resistance of the friction material, or increasing the amount of hydraulic fluid supplied for cooling, implementing these measures would either make the apparatus complex or increase costs, neither of which is desirable. It is also possible that when the heat load becomes high, the neutral control may be temporarily cancelled in order to protect the friction apply device. This would negate the improvement in fuel efficiency improvement that the apparatus was designed to achieve.